tests/Dockerfile: add "REPLACE_ME_GO_VERSION" back

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

tests/Dockerfile

@@ -15,7 +15,7 @@ RUN dnf check-update || true \
 ENV GOROOT /usr/local/go
 ENV GOPATH /go
 ENV PATH ${GOPATH}/bin:${GOROOT}/bin:${PATH}
-ENV GO_VERSION 1.10.1
+ENV GO_VERSION REPLACE_ME_GO_VERSION
 ENV GO_DOWNLOAD_URL https://storage.googleapis.com/golang
 RUN rm -rf ${GOROOT} \
   && curl -s ${GO_DOWNLOAD_URL}/go${GO_VERSION}.linux-amd64.tar.gz | tar -v -C /usr/local/ -xz \